Dispersion Mechanism And Visualization In Flow Channels Of Co-rotating Non-twin Screw Kneading Blocks Dispersion Mechanism And Visualization Experiment

With the continuous development of extruders,the conventional equal-speed twin screw extruder has been widely used in all aspects of polymer processing,however,the traditional twin-screw extruder lacks spatial transformation during the melt mixing process due to the same configuration of both screws,resulting in weak interfacial growth and reorientation,which limits the further enhancement of mixing ability.In order to overcome this defect and enhance the mixing effect,a new dual-speed non-twin screw was developed,which breaks the symmetric structure of the conventional twin-screw and achieves topological chaotic mixing during polymer processing,and significantly improves the melt mixing efficiency.In this thesis,numerical simulations,visualization experiments and PIV testing techniques were combined together to further investigate the dispersive mixing mechanism of the kneading blocks of such novel screw configurations.The three-dimensional models of the kneading blocks of the conventional twin screws and non-twin screws were established,and the mixing characteristics of the two blocks with a stagger angle of zero were investigated deeply using the fluid dynamics software POLYFLOW.The numerical simulation results include velocity field,pressure fluctuation,flow fluctuation,and particle tracking are solved using a self-designed post-processing program to characterize the mixing performance of the dual-speed non-twin kneading blocks from several aspects.The numerical simulation also shows that the dual-speed non-twin kneading block has better mixing performance.We investigated the mixing mechanism of the non-twin 0° kneading block using a selfdeveloped transparent extruder with full visualization.For the first time,the phenomenon of"periodic fluctuation" of the non-twin 0° kneading block during the extrusion process was captured,and the results showed that the "periodic fluctuation" accelerated the rupture of the large scale tracer,but made the tracer pass through the block rapidly,resulting in the degradation of the mixing performance.The first red sand method was used to obtain the residence time distribution of the non-twin 0° kneading block.The results show that the more groups of kneading blocks and the faster the speed,the more intense the "periodic fluctuations"are,and the addition of threaded elements reduces the intensity of the periodic fluctuations,and the flow field state is only related to the current rotational state of the screw.For the first time,the effect of the pinch block on the outlet flow was characterized by the filming method,and the results showed that the pinch block configuration and the screw speed had a greater effect on the instantaneous flow fluctuations at the outlet.The exit film pulling method was used to characterize the dispersion and mixing capacity within the kneading block.The material outlet was equipped with a film and pulling device,and particle size statistics were performed after collecting tracer dispersion images using a camera.The higher the screw speed and the smaller the feeding volume,the better the dispersion and mixing effect;in a group of kneading block,the best dispersion effect was achieved in the non-twin 90°/45° kneading block,followed by the dispersion and mixing ability in the non-twin 180°/90° and 120°/60° kneading blocks,and the worst dispersion and mixing ability in the non-twin 60°/30° kneading block.When three groups of kneading blocks,the dispersion and mixing ability of the non-twin speed 90°/45° kneading block is the best,and the dispersion and mixing ability of the non-twin speed 0° kneading block is the worst.The mixing and dispersing ability of the non-twin speed kneading block was better than that of the conventional kneading block.From the multi-factor ANOVA,the number of kneading block groups,feeding volume and misalignment angle have a significant effect on material dispersion;from the interaction can be seen,the screw speed and feeding volume on the average particle size and material dispersion uniformity play a key role.There are 100 figures,30 tables and 52 references.